1. Field of the Invention
The present invention relates to an apparatus equipped with a motor and a driving method for the motor, and in particular, relates to driving of a direct current (DC) motor as a driving source.
2. Description of the Related Art
Apparatuses equipped with a motor include, for example, an image reading apparatus (scanner), recording apparatus (printer), and multifunction peripheral having a reading function and a recording function. Such apparatuses suppress working sound by using a DC motor.
An image reading apparatus (scanner) is required to provide high read resolution, as well as suppression of working sound. On the other hand, such a scanner executes a mode of reading in low resolution of, for example, 75 dots per inch (dpi) and also a mode of reading in high resolution like 9600 dpi. To handle such a wide range of resolution, it is necessary for a motor that moves a carriage to be provided for traveling speeds from a low speed to a high speed.
If a scanner in which a carriage whose driving source is a DC motor is operated should realize constant low-speed movement to implement high-resolution reading as described above, feedback control needs to be performed in a control period appropriate for a mechanism unit thereof. For this purpose, it is necessary to know the position of the carriage with high precision, which requires a high-resolution encoder. Thus, if the carriage is moved at a low speed, a signal of the encoder changes less when compared with a case of a high speed so that speed fluctuations due to an influence of cogging or the like cannot be suppressed. If the carriage speed is faster than a time necessary to read pixels of one line (hereinafter, referred to as one read line) due to the speed fluctuations, the carriage will pass through the line and move to the next read line even though the line to be read has not been read. This results in line shifts in an output image. Similarly, if the carriage speed is slower than the time necessary to read one read line, an overflow of read data occurs due to overexposure or dark current noise increases due to delayed lights-out of a light source of a read sensor. This results in degraded quality of an output image.
To avoid the above issues, the DC motor needs to be controlled by an encoder of resolution appropriate for reading a document in read resolution. Particularly when images are read in various resolutions and the range of resolution is wide, it is necessary to devise some contrivance to handle such a situation.
In the past, for example, like Japanese Patent Application Laid-Open No. 2001-99850, providing a high-resolution encoder for low speed, in addition to an encoder (high-speed encoder) held by a conventional scanner, to use the two encoders for different purposes has been discussed to realize such contrivance.
When mounting a DC motor equipped with a high-resolution encoder to read a document in high resolution, apparatuses on which such a DC motor can be mounted are limited due to factors such as the physical size thereof and cost. This also applies to the method discussed in Japanese Patent Application Laid-Open No. 2001-99850 and it is difficult to mount such a DC motor on a low-priced small-sized image processing apparatus.
For the above reason, there are many cases where only encoders having resolution that is insufficient to satisfy specification requirements thereof can be mounted on image processing apparatuses. Thus, even if an attempt is made to move the carriage at a constant low speed, precise speed information cannot be acquired so that speed fluctuations of the carriage cannot be followed. This results in lower quality of an output image such as pixel shifts and is a factor that prevents mounting of a DC motor on an image processing apparatus that supports reading of a high-resolution image.
To deal with this issue, performing control so that a step movement is made for reading by stopping after a speed servo for one read line being performed when a document image is read in high resolution is assumed, instead of moving the carriage at a constant speed throughout the entire region of an image. That is, control that enables driving with less amount of rotation each time a DC motor is driven is assumed.
However, when the above control is performed, a phenomenon in which the motor stops less precisely due to torque fluctuations mainly caused by the DC motor occurs. This is a phenomenon (hereinafter, referred to as swing-back) in which the stop position shifts forward while the motor stops after the end of a servo region. If the amount of shift (hereinafter, referred to as an overrun) between the target stop position and the actual stop position is significant, the precision with which an operation or processing is performed by an apparatus cannot be improved.